This means that compared to generic metals, even light reinforced composites structures can handle enormous amounts of load or strain. Anchors that are too light may not dig in or hold well enough to perform their function. Cost is also a factor when less expensive, heavy anchors do a good job of holding a vessel in place.
Image Credit: Marine Insight. How anchors work. Modern anchors have a chain near the anchor followed by lighter cable or rope up to the vessel. The anchor chain helps give added weight so the anchor can be set with horizontal force to dig in and stay put.
Recommended length of rope and chain is 7x depth. Image Credit: West Marine. Types of anchors: time and conditions. There are many different variations of anchors. They have evolved heavily over time, but the main concern is how they perform in different bottom conditions.
Assessing those conditions is the most important part of proper anchor function. Then take both lines to the bow of the boat. This allows the boat to swing around in a relatively small arc, yet will allow the boat to pull against an anchor without causing it to reset when wind or current change. Anchors need to develop enough resistance in the seabed to withstand the environmental forces on the boat—the wind and the waves. An anchor's ability to develop resistance is entirely dependent on its ability to engage and penetrate the seabed.
We have participated in several anchor tests, and despite varying results, there always seems to be one undeniable conclusion: the selection of a suitable bottom for anchoring is a much more critical factor than the design of the anchor. So how do you choose the right anchor design?
You must take expected bottom conditions into account. Here is an analysis of potential options, based on the seabed. Fortress Anchors are highly regarded for their holding ability in soft bottoms.
Fine-grained sand is relatively easy for anchors to penetrate and offers consistently high holding power and repeatable results. Most anchors will hold the greatest tension in hard sand. Best in sand are the lightweight Danforth-style anchors like the West Marine Traditional and Fortress anchors. Mud has low shear strength, and requires anchor designs with a broader shank-fluke angle and greater fluke area.
This allows the anchor to penetrate deeply to where the mud has greater shear strength, and also presents more surface area in the direction of pull. Mud is frequently only a thin layer over some other material, so anchors that can penetrate through the mud to the underlying material will hold better. Fortress anchors have superior holding power in mud, because they can be converted to a broad fluke angle. Offshore structures are often deployed for long periods, where they remain in a single location with minimum movement.
Due to the expensive and extremely sensitive nature of the equipment used by oil rigs and energy harvesters, they cannot afford to move by more than a few millimetres. Also, the risk of damaging important components or causing oil leaks require these structures to be anchored in such a manner that underwater currents and small waves are not able to easily dislodge and move the entire setup.
Permanent anchors are used in this regard to restricting the movement of such structures. Also, a specialized class of tug boats known as Anchor Handling Tugs AHTS often use this anchor line to tow these semi-submersible structures from one location to another for deployment. In general, such anchors are not meant to be frequently moved and spend extended periods at the same spot.
However, when the time comes to move the floating structure, the anchors often need to be reeled back on the surface or towed using an AHTS. In such cases, to dislodge the anchor, a trip line is attached to the head and is connected to the structure. In the event of the anchor head being stuck to the seafloor, the trip line can be used to provide an additional force to move the anchor.
Sometimes, explosives or small, controlled charges are used to disturb the bed of the ocean. The anchor is then hauled back up to the surface. In addition to these two methods, some types of anchors use detachable heads that can be left on the seafloor after the operations are completed, and when it is time to move the structure.
The anchor chain and stock are reeled on to the structure leaving the head behind. The problem with such a type of anchor is that it leads to pollution of the ocean floor, especially if the metals are toxic to marine flora and fauna.
Besides, there is a risk of accidentally disengaging the head from the rest of the anchor while the structure is still in operation. The most common method of dislodging the anchor is by using a trip line aided with contained charges deployed close to the ocean bed. It is important to finalize the number of anchors to be used to tether any structure.
From the study of mechanics of a body, it is known that using a three-point anchoring mechanism, any structure can be completely held stationary. This is because forces from any direction can always be countered by the alignment of the anchors in such a system. The common type of anchors used in such permanent deployment situations is the mushroom, auger, high-holding, and deadweight methods. Mushroom anchors, as the name suggests, are shaped like inverted mushrooms, with the head being laid in the sea or ocean bed.
This style of anchor utilizes its weight, suction power and relative friction between the bed and anchor head to keep itself firmly attached to the strata of the ocean floor. However, it only works in conditions when mud, silt or sand are prevalent on the floor of the ocean. Other materials such as rock and sand are not able to provide the adhesion required to keep the anchor firmly attached to the ocean floor.
The science behind how this system works is that the anchor uses a derived version of the Archimedes Principle on soft, granular or viscous media such as mud and sand. Since these materials cannot generally hold up the weight of an anchor these weights can reach up to several tonnes for ships with abnormally high displacements , they allow the head to sink in until it has displaced enough strata material to equal its weight.
Due to the sheer size of such anchors, these could easily account for several meters of depth in the ocean or seafloor. They can resist almost all types of wave motions and even the most severe of storms. To remove them, the sand or mud surrounding the anchor is dislodged, until the adhesive attraction between the head and the strata material is weak enough to be broken by the anchor hauling force provided by the motors on the structure.
While the strength provided by these anchors makes them very useful in restricting motion, they can only work in regions where the floor of the ocean or sea provides sufficient suction to drag down the anchor. This makes them ideal in regions close to beaches or lagoons. The auger type of anchors makes use of the physics behind the high retention power of screw design, and their ability to remain locked in a position for extended periods. These anchors consist of large threaded heads that are drilled into the bed of the sea or ocean where the structure is to be installed.
Often instead of directly being driven into the ocean floor, a casing is first attached to the bottom of the ocean, with grooves cut into it. The casing and screw heads are often made from titanium or similar alloys and materials resistant to rusting and corrosion from exposure to water and underwater organisms. The reason for using titanium is that it is ideal for creating strong and unreactive components such as riser joints in oil rigs. However, the relative abundance of titanium and the extensive treatment of the materials used in the production of this type of anchor tends to make the process of manufacture and set up expensive.
Another problem that arises while setting up the anchor is that ready access to the case and screw head must be provided, since perfect alignment is required, and any errors in drilling the ocean or sea bed can lead to damage to the equipment. Thus, this type of permanent anchor is mainly used in regions that are shallow and located close to the shore or have low-tides that make it possible to access the casing and head. Issues can also arise when the bottom stratum is made of soft pliable materials such as mud, silt or sand.
Since the screw works on the principle of friction between the casing and head, such materials do not generate sufficient traction for the screw to properly grip the ocean or sea bed. The screw and its casing will continuously swivel without actually being able to anchor the structure. However, despite so many restrictions on the location and deployment of this type of anchor, it is considered to be one of the strongest methods of permanently anchoring any structure. In operating environments where all the basic requirements are met, these anchors are commonly found.
These anchors are used in the oil and gas industry to tether large semi-submersible structures or to hold down underwater pipelines running across the ocean bed or seafloor. Such anchors are considerably larger and heavier than their counterparts. To be declared as a high holding anchor, they must exhibit strength and retention capabilities equal to twice the normal values of conventional anchors.
The conventional anchor considered for reference purposes must be the same weight as the HHP anchor being tested. To achieve this tag, three tests must be conducted successfully, and the anchor must be tried in a minimum of three different types of soil. Compared to the previous two types of permanent anchors, this classification ensures that the anchor functions in any type of condition. Similar to HHP anchors, Super High Holding Power is a classification that guarantees that the tested anchor can withstand a minimum force equivalent to four times that of a normal anchor weighing the same.
If your boat engine fails, a well-set anchor will keep wind or current from drifting your disabled boat onto a shoal or ashore, where it could be damaged. The most-common pleasure boat anchor type are the fluke often called a Danforth , and the plow or scoop anchor. Use anchor manufacturer guidance to determine which size anchor is appropriate for your boat.
You can learn more about different anchor options and how to find the right size for your boat by reading Types of Boat Anchors: How to Choose the Right Size. You want to make sure that the anchor is set and not dragging on the bottom. You can sight on two landmarks on shore, or use electronics such as GPS, a chart plotter, or a depth finder to sound an alarm if the boat is moving.
If the wind, current or tide changes and causes the boat to swing over the anchor, it may reset itself on the bottom. Read Next: How to Dock a Boat. Skip to content.
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